Liquid crystal displays have been widely used in monitors of personal computers and mobile telephones, TV sets and so on because of having a number of merits such as running at low voltage, requiring little electricity and being available in downsized and thinned modes. These liquid crystal displays have been proposed in various modes depending on the aligned states of liquid crystal molecules in liquid crystal cells. Among all, the TN mode, wherein liquid crystal molecules are aligned at a twist angle of about 90° from the lower substrate of a liquid crystal cell toward the upper substrate, has been mainly employed.
A liquid crystal display is usually made up of a liquid crystal cell, an optically compensatory sheet and a polarizer. As the optically compensatory sheet, which is employed to relieve image coloration or enlarge viewing angle, use has been made of films obtained by coating a stretched birefringent film or a transparent film with liquid crystals. For example, Patent Document 1 discloses a technique wherein an optically compensatory sheet, which is obtained by coating discotic liquid crystals on a triacetylcellulose film and then fixing by orientation, is applied to a TN mode liquid crystal cell to thereby enlarge the viewing angle. In a liquid crystal display for a wide-screen TV likely watched from various angles, however, accurate viewing angle-dependency is required and this requirement cannot be fulfilled by using the techniques as discussed above. Thus, studies have been made on liquid crystal displays in modes different from the TN mode, for example, the IPS (in-plane switching) mode, the OCB (optically compensatory bend) mode and the VA (vertically aligned) mode. Among all, the VA mode has attracted public attention as a liquid crystal display for TV since it establishes a high contrast and can be produced at a relatively high yield.
However, the VA mode suffers from a problem that though it provides almost complete black display in the normal line direction of the panel, light leakage arises in observing the panel from an angle and thus the viewing angle is narrowed. To solve this problem, there has been proposed a method of relieving the light leakage by the combined use of a first phase contrast plat having a positive refractive index anisotropy (nx>ny=nz) with a second phase contrast plat having a negative refractive index anisotropy (nx=ny>nz) (see, for example, Japanese Patent No. 3027805). It has been also proposed to improve the viewing angle characteristics of a VA mode liquid crystal display by using an optically biaxial phase contrast plate (nx>ny>nz) (see, for example, Japanese Patent No. 3330574), wherein nx, ny and nz respectively stand for the refractive indexes in the X-axis, Y-axis and Z-axis directions. The X-axis direction corresponds to the direction showing the maximum refractive index in the in-plane direction of the phase contrast plate, the Y-axis direction corresponds to the direction perpendicular to the X-axis direction in plane, and the Z-axis direction corresponds to the thickness direction perpendicular to the X-axis and Y-axis directions as described above.
By these methods, however, light leakage within a certain wavelength range (for example, green light at around 550 nm) can be exclusively relieved and no attention is paid to light leakage in other wavelength ranges (for example, blue light at around 450 n, or red light at around 650 nm). Therefore, so-called color shift (i.e., coloration in blue or red in the case of, for example, looking black display at an angle) cannot be improved thereby.
To obtain a definite image in vivid colors, a phase plate for liquid crystal displays should have a birefringent layer being optically even on the whole face and show no change in the optical characteristics due to changes in temperature or humidity. As materials for these phase plates, there have been proposed to use thermoplastic resin films as reported in Japanese Patent No. 3273046.
However, color shift as discussed above cannot be improved even by using such films as reported in Japanese Patent No. 3273046.